The influence of hydrological dynamics on iron mineral transformation

Water Alliance

Redox driven iron mineral transformation has become an increasing attention due to its relevance for a number of biogeochemical reactions being involved in this process. In many anoxic environments as well as at anoxic-oxic interfaces such as groundwater aquifers, wetlands, marine sediments or coastal ecosystems the interaction between dissolved sulfide(S(-II) and ferric minerals plays a prominent role. The process leads to metastable iron sulfide minerals formation, such as nano mackinawite, which further transfers to pyrite due to its thermodynamic instability. However, the rate of pyrite formation and its pathways differ between redox conditions. In previous work, we found pyrite formation to occur rapidly (in 7 days) under conditions where initially added S(-II) was rapidly consumed but iron oxides are still present (Hellige et al, 2012; Peiffer, et al, under review; Wan, PhD thesis).The crystal growth rate of pyrite is then constrained by the reactivity of iron oxides/reactive surface sites (Peiffer et al. under review). In contrast, no pyrite formation was observed within at least 1 month under conditions where S(-II) is still present and most initially added iron oxides were reduced(Wan, PhD thesis).
Given these differences in reactivity and particular product formation depending on the S(-II) availability we hypothesize a strong effect of geochemical gradients on product formations as being triggered by hydrological dynamics. This will be of particular relevance under varying redox conditions switching along with the variation of groundwater flow rate. To test this hypothesis, we are planning laboratory and field studies in combination with in-situ Raman spectroscopy as well as Mössbauer spectroscopical studies. The iron oxides will be exposed to a sulfide containing solution, the flow rate of which will be carefully adjusted and the mineral transformation will be followed along the geochemical gradients. Experiments will be run under different flow conditions. We expect preservation of iron sulfide mineral/mackinawite in the runs with fast flow rate and significant transformation from iron sulfide to pyrite with slow flow rate. We are designing the experiments with ferric oxides coated carriers(glass plates), which will react to sulfide solution with different flow rates.